Once Dropbox throws you an error that says Unable to monitor filesystem. Please run: echo 100000 | sudo tee /proc/sys/fs/inotify/maxuserwatches and restart Dropbox to correct the problem.” you’d better adjust settings in sysctl.conf file, to keep changes after reboot.

Open system config (as superuser, of course) in your favorite text editor(mine is vi):

vi /etc/sysctl.conf

Add this line to the end of the file:

fs.inotify.max_user_watches = 100000

Save the file (Shift-ZZ, if you are new to Vi).

sysctl -p

To apply the settings and then just restart Dropbox.

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